This invention relates generally to a rotor assembly for a stepping motor used in an electronic timepiece, and more particularly to an improved stepping motor rotor assembly which is less costly to manufacture.
A quartz analog wristwatch employs a small stepping motor consisting of a stator with coil periodically energized with electrical current pulses of alternating polarity to step the motor and drive the wristwatch hands through a gear train. The rotor of the stepping motor includes a drive member such as a gear pinion, a permanent magnet, usually having two poles, and a pinion shaft with journals rotatably mounted in the watch movement. The permanent magnet is often in the shape of a small cylinder or ring with a central hole through it for the pinion shaft.
The permanent magnets, typically anistrophic rare earths such as samarium cobalt, are extremely brittle and difficult machine. Because of the small size of the rotor, close tolerances are required in the stepping motor. Various improvements have been suggested to reduce breakage of the magnets and to reduce cost of manufacturing a stepping motor rotor. The possibility of breakage increases when designs require a press fit of the magnet material to the rotor pinion shaft, or when compressive forces on the outer diameter of the magnet are necessary to hold it in place on the rotor. Protective bushings have been used to reduce breakage.
A desirable characteristic of a stepping motor rotor is that it have a low moment of inertia about its axis of rotation. Since the moment of inertia varies as the square of the radius of rotation, the use of unnecessary protective bushings between the inner diameter of the magnet and the rotor shaft or the use of metallic shells encasing the outer diameter of the magnet are to be avoided.
Prior art rotor assemblies with plastic internal bushings disposed between the shaft and the magnet are shown in U.S. Pat. Nos. 4,206,379--Onda; 3,953,752--Bannon; 2,488,729--Kooyman; and 4,340,560--Migeon assigned to Applicant's assignee.
An example of a stepping motor rotor with permanent magnet held clamped axially between metal bushings which are press fit with an interference fit on the rotor shaft is seen in U.S. Pat. No. 4,035,677--Kusayama. In the Kusayama Patent, centering of the magnet is performed by loosely fitting the inner diameter of the magnet on the bushings, which requires very close tolerances and expensive finishing of the inside hole diameter of the brittle magnet material. Constructions are shown where the inner hole diameter of the permanent magnet is finished and pressed directly onto the rotor shaft in U.S. Pat. No. 3,943,698 which uses a reinforcing plate of stainless steel or other non-magnetizable material bonded to the magnet to prevent cracking the brittle material.
A rotor assembly which employs a radial wall and outer protective metal sheath into which the magnet is press fit on its outer diamter is disclosed in U.S. Pat. No. 4,095,129--Tanai. Although this permits forming the inner diameter of the rare earth magnet by a rough drilling process, the outer metallic protecting sheath both increases the moment of inertia and offers the possibility of breakage by compressive stresses due to the press fit on the outer diameter of the magnet.
It would be desirable to provide an improved rotor assembly with a low moment of inertia and a method of holding the permanent magnet with reduced stress on the brittle magnet. It would also be desirable to provide a design permitting improved manufacturing methods which would reduce the cost of the rotor assembly.
Accordingly, one object of the present invention is to provide an improved stepping motor rotor assembly and method of manufacture for a low cost, stepping motor rotor.
Another object of the invention is to provide a simplified design for a stepping motor rotor with lower movement of inertial.